The 3rd Generation Partnership Project (3GPP) work on Licensed-Assisted Access (LAA) intends to allow Long Term Evolution (LTE) equipment to also operate in the unlicensed radio spectrum. Candidate bands for LTE operation in the unlicensed spectrum include 5 GHz and 3.5 GHz, among others. The unlicensed spectrum can be used as a complement to the licensed spectrum, but also allows completely standalone operation.
For the case of unlicensed spectrum used as a complement to the licensed spectrum, devices connect in the licensed spectrum (e.g., via a primary cell (PCell)) and use carrier aggregation (CA) to benefit from additional transmission capacity in the unlicensed spectrum (e.g., via a secondary cell (SCell)). The CA framework allows a device to aggregate two or more carriers, with the condition that at least one carrier (or frequency channel) is in the licensed spectrum and at least one carrier is in the unlicensed spectrum. In the standalone (or completely unlicensed spectrum) mode of operation, one or more carriers are selected solely in the unlicensed spectrum.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing, transmission power limitations, and/or imposed maximum channel occupancy time. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so-called listen-before-talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Due to the centralized coordination and dependency of terminal devices on the base-station (e.g., evolved NodeB (eNB)) for channel access in LTE operation, and imposed LBT regulations, LTE uplink (UL) performance is especially hampered. UL transmission is becoming more and more important with user-centric applications and the need for pushing data to cloud.
Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi” and allows completely standalone operation in the unlicensed spectrum. Unlike in LTE, Wi-Fi terminals can asynchronously access the medium and thus show better UL performance characteristics, especially in congested network conditions.
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (DL) and Discrete Fourier Transform (DFT)-spread OFDM (also referred to as Single-Carrier Frequency Division Multiple Access (SC-FDMA)) in the UL.
FIG. 1 illustrates an example LTE DL physical resource. As shown in FIG. 1, the basic LTE DL physical resource can be seen as a time-frequency grid where each resource element (e.g., resource element 10) corresponds to one OFDM subcarrier during one OFDM symbol interval. The UL subframe has the same subcarrier spacing as the DL, and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the DL.
FIG. 2 illustrates an example of the LTE time-domain structure. In the time domain, LTE DL transmissions are organized into radio frames (such as radio frame 20) of 10 ms. Each radio frame 20 consists of ten equally-sized subframes of length Tsubframe=1 ms, as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
The resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in the time direction (i.e., 1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
DL transmissions are dynamically scheduled (i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current DL subframe). This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The DL subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, for example, the control information.
FIG. 3 illustrates an example DL subframe. More particularly, FIG. 3 illustrates a DL system with CFI=3 OFDM symbols as control. In the example of FIG. 3, the reference symbols shown are the cell specific reference symbols (CRS), which are used to support multiple functions, including fine-time and frequency synchronization and channel estimation for certain transmission modes.
UL transmissions are dynamically scheduled (i.e., in each DL subframe the base station transmits control information about which terminals should transmit data to the base station in subsequent subframes, and upon which resource blocks the data should be transmitted). The UL resource grid is comprised of data and UL control information in the Physical Uplink Shared Channel (PUSCH), UL control information in the Physical Uplink Control Channel (PUCCH), and various reference signals such as demodulation reference signals (DMRS) and sounding reference signals (SRS). DMRS are used for coherent demodulation of PUSCH and PUCCH data, whereas SRS is not associated with any data or control information but is generally used to estimate the UL channel quality for purposes of frequency-selective scheduling.
FIG. 4 illustrates an example UL subframe. Note that UL DMRS and SRS are time-multiplexed into the UL subframe, and SRS are always transmitted in the last symbol of a normal UL subframe. The PUSCH DMRS is transmitted once every slot for subframes with normal cyclic prefix, and is located in the fourth and eleventh SC-FDMA symbols.
From LTE Release 11 onwards, DL or UL resource assignments can also be scheduled on the Enhanced Physical Downlink Control Channel (EPDCCH). For Release 8 to Release 10, only the Physical Downlink Control Channel (PDCCH) is available. Resource grants are user equipment (UE)-specific, and are indicated by scrambling the Downlink Control Information (DCI) Cyclic Redundancy Check (CRC) with the UE-specific Cell Radio Network Temporary Identifier (C-RNTI) identifier. A unique C-RNTI is assigned by a cell to every UE associated with it, and can take values in the range 0001-FFF3 in hexadecimal format. A UE uses the same C-RNTI on all serving cells.
In LTE, the UL access is typically controlled by the eNB (i.e., scheduled). In this case, the UE would report to the eNB when data is available to be transmitted, for example by sending a Scheduling Request (SR) message. Based on this, the eNB would grant the resources and relevant information to the UE in order to carry out the transmission of a certain size of data. The assigned resources are not necessarily sufficient for the UE to transmit all the available data. In such a case, the UE may send a buffer status report (BSR) control message in the granted resources to inform the eNB about the correct size and updated size of the data waiting for transmission. Based on that, the eNB would further grant the resources to carry on with the UE UL transmission of the corrected size of data.
More particularly, every time new data arrives at the UE's empty buffer, the following procedure should be performed. Using the PUCCH, the UE informs the network that it needs to transmit data by sending a SR indicating that it needs UL access. The UE has a periodic timeslot for SR transmissions (typically on a 5, 10, or 20 ms interval). Once the eNB receives the SR request bit, it responds with a small “UL grant” that is just large enough to communicate the size of the pending buffer. The reaction to this request typically takes 3 ms. After the UE receives and processes its first UL grant (which takes about 3 ms), the UE typically sends a BSR, which is a Medium Access Control (MAC) Control Element (CE) used to provide information about the amount of pending data in the UL buffer of the UE. If the grant is big enough, the UE sends data from its buffer within this transmission as well. Whether the BSR is sent also depends on conditions specified in 3GPP TS 36.321 v12.1.0 (2014-03), “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification,” Release 12. The eNB receives the BSR message, allocates the necessary UL resources, and sends back another UL grant that will allow the device to drain its buffer.
In total, about 16 ms (plus time to wait for a PUCCH transmission opportunity) of delay can be expected between data arrival at the empty buffer in the UE and the reception of this data in the eNB.
In case the UE is not Radio Resource Control (RRC) connected in LTE, or has lost its UL synchronization since it did not transmit or receive anything for a certain time, the UE would use the random access (RA) procedure to connect to the network, obtain synchronization, and also send the SR. If this is the case, the procedure until the data can be sent would take even longer than the SR transmission on PUCCH.
In the LTE system, the transmission formats and parameters are controlled by the eNB. Typically, the DCI contains: resources allocated for UL transmission (including whether frequency hopping is applied); modulation and coding scheme; redundancy versions; new data indicator; transmit power control command; information about DMRS; the target carrier index (in cases of cross-carrier scheduling); and other applicable control information on UL transmissions. The DCI is first protected by 16-bit CRC. The CRC bits are further scrambled by the assigned UE identity (e.g., C-RNTI). The DCI and scrambled CRC bits are further protected by convolutional coding. The encoded bits are transmitted from the eNB to UE using either PDCCH or EPDCCH.
Semi-Persistent Scheduling (SPS) is similar to pre-scheduling in that the UE modem is granted radio resources periodically. The main difference is that there is no explicit grant signal sent every time the UE modem is granted resources. Instead, the eNB sends a long-lasting grant that allows the UE modem to keep track of when it is granted resources and use that time/frequency slot for sending data.
Instant Uplink Access (IUA) was discussed in the 3GPP latency reduction study item. IUA is a form of pre-scheduling to allow transmission of data without explicit SRs. IUA is an enhancement of the SPS framework that introduces a new UE condition: “Do not transmit using the grant unless there is data in buffer.”
In the current LTE framework, a UE with an UL grant is forced to send something. Even if the UE has no UL data, the UE would send padding. In addition, the lowest SPS period is 10 subframes. With fast UL, two modifications are done to SPS to enable IUA. First, it allows an SPS period down to 1 subframe (or transmission time interval (TTI)). Second, it allows a skip-padding configuration (i.e., the UE does not need to transmit in the granted UL resources if it does not have data for that subframe).
In the current LTE specifications, the SPS and IUA features are defined only for the primary cell (PCell). The configuration and commands of SPS and IUA are also defined and performed on a per-UE basis.
In typical deployments of WLANs, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a Clear Channel Assessment (CCA), and a transmission is initiated only if the channel is declared Idle. If the channel is declared Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
FIG. 5 illustrates an example of the LBT mechanism in Wi-Fi. After a Wi-Fi station A transmits a data frame to a station B, station B transmits the acknowledgement (ACK) frame back to station A with a delay of 16 μs. The ACK frame is transmitted by station B without performing a listen-before-talk (LBT) operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as Distributed Inter-frame Space (DIFS)) after the channel is observed to be occupied before assessing again whether the channel is occupied. Thus, a station that wishes to transmit first performs a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle, then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
In the basic protocol described above, when the medium becomes available multiple Wi-Fi stations may be ready to transmit. This can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW]. The default size of the random backoff contention window, CWmin, is set in the IEEE specifications. Note that collisions can still occur even under this random backoff protocol, for example when there are many stations contending for channel access. Hence, to avoid recurring collisions, the backoff contention window size CW is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, also set in the IEEE specifications. When a station succeeds in a transmission without collision, it resets its random backoff contention window size back to the default value CWmin.
FIG. 6 illustrates an example of LBT in European Telecommunications Standards Institute (ETSI) EN 301.893. For a device not utilizing the Wi-Fi protocol, the ETSI standard EN 301.893, v1.7.1 provides the following requirements and minimum behavior for the load-based CCA.
A first requirement is that before a transmission or a burst of transmissions on an Operating Channel, the equipment shall perform a Clear Channel Assessment (CCA) check using “energy detect.” The equipment shall observe the Operating Channel(s) for the duration of the CCA observation time which shall be not less than 20 μs. The CCA observation time used by the equipment shall be declared by the manufacturer. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in the fifth requirement described below. If the equipment finds the channel to be clear, it may transmit immediately (see the third requirement described below). This is shown in the example of FIG. 6 at time intervals 1 and 2. At time interval 1, the equipment performs a CCA check as described above. Having found the channel to be clear, the equipment transmits immediately during time interval 2.
A second requirement is that if the equipment finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an Extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that need to be observed before initiation of the transmission. The value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer (see clause 5.3.1 q)). The counter is decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the equipment may transmit.
This is shown in the example of FIG. 6 at time interval 3. At the beginning of time interval 3, the equipment performs a CCA check and finds that the channel is occupied. Thus, the equipment performs an Extended CCA check as described above. In the example of FIG. 6, the value of N is initialized by N=3. The counter N is decremented every time a CCA slot is considered to be unoccupied. In the example of FIG. 6, the first channel CCA slot in the Extended CCA check is determined to be unoccupied, so the counter is decremented from 3 to 2. The second channel CCA slot in the Extended CCA check is determined to be occupied, so the counter is not decremented from 2. The third channel CCA slot in the Extended CCA check is determined to be unoccupied, so the counter is decremented from 2 to 1. Similarly, the fourth channel CCA slot in the Extended CCA check is determined to be unoccupied, so the counter is decremented from 1 to 0. When the counter reaches zero, the equipment transmits during time interval 4.
A third requirement is that the total time that an equipment makes use of an Operating Channel is the Maximum Channel Occupancy Time which shall be less than (13/32)×q ms, with q as defined in the second requirement described above, after which the device shall perform the Extended CCA described in the second requirement above.
A fourth requirement is that the equipment, upon correct reception of a packet which was intended for this equipment, can skip CCA and immediately proceed with the transmission of management and control frames (e.g., ACK and Block ACK frames). This is shown in the example of FIG. 6 during time interval 5. A consecutive sequence of transmissions by the equipment, without it performing a new CCA, shall not exceed the Maximum Channel Occupancy Time as defined in the third requirement described above. For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.
A fifth requirement is that the energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter. For a 23 dBm e.i.r.p. transmitter, the CCA threshold level (TL) shall be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA threshold level TL shall be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.).
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue. It also allows spectrum efficiency to be maximized. The spectrum allocated to LTE is limited, however, and cannot meet the ever-increasing demand for larger throughput from applications and/or services. Therefore, Release 13 LAA extended LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. LTE therefore needs to consider the coexistence issue with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in unlicensed spectrum as in licensed spectrum can seriously degrade the performance of Wi-Fi, as Wi-Fi will not transmit once it detects the channel is occupied.
FIG. 7 illustrates an example of LAA to unlicensed spectrum using LTE carrier aggregation. One way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 7, a wireless device 110 (e.g., a UE) is connected to a PCell 705 in the licensed band and one or more secondary cells (SCells) 710 in the unlicensed band. As used herein, a secondary cell in unlicensed spectrum is referred to as a LAA secondary cell (LAA SCell). In the case of standalone operation (as in MulteFire), no licensed cell is available for UL control signal transmissions.
The combination of the LBT and maximum transmission burst duration functionalities of LAA/MulteFire implies that LTE reference signals are not guaranteed to be transmitted with a fixed periodicity. To support synchronization, frequency estimation, and radio resource management (RRM) measurements, the discovery reference signal/subframe (DRS) is periodically transmitted and contains the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), Cell-Specific Reference Signal (CRS), and Channel State Information Reference Signal (CSI-RS) for LAA, and also the Physical Broadcast Channel (PBCH) and Session Information Block (SIB) transmission for MulteFire. Due to LBT constraints, DRS transmission cannot be guaranteed in a particular time instance. Hence, the DRS can be transmitted within a window specified by the DRS Measurement Time Configuration (DMTC).